1. Field of the Invention
The present invention relates to a power supply device for and a power supply method of supplying power to electronic circuits by a battery, and electronic equipment provided with various electronic circuits, and a battery that supplies power to the electronic circuits, and specifically to a power supply device, a power supply method and electronic equipment wherein power is supplied to electronic circuits from a battery with decreased recharging capacity.
2. Background of the Invention
Typically, electronic equipment consists of electronic circuits that are driven by a battery or other power supply means. For example, portable electronic equipment, e.g. portable terminals, are driven by a primary battery or a rechargeable secondary battery.
Secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries and the like, and these nickel-cadmium batteries and nickel-hydrogen batteries have the advantages of a long history of practical use, high safety and low cost. Because of this, secondary batteries of nickel-cadmium batteries and nickel-hydrogen batteries have been used conventionally for driving electronic equipment.
However, nickel-cadmium batteries and nickel-hydrogen batteries have a disadvantage in that the so-called memory effect wherein voltage drops occur depending on their method of usage. The memory effect is an effect wherein the recharging capacity drops, a phenomenon that occurs for example when a deep discharge is performed after repeated shallow charging and discharging cycles, and then the voltage after discharge drops in two stages.
To explain this more specifically, the memory effect occurs particularly in sealed nickel-cadmium batteries or the like, when cycles of shallow discharging(discharging that ends before the capacity is completely regained) and recharging are repeated, the capacity is lost, leading to reversible deterioration. For example, as shown in FIG. 1, if cycles of partially discharging and recharging a battery are performed repeatedly, the voltage and capacity that can be obtained drop gradually with the number of cycles (curve 2 shows repeated cycles). If the battery is completely discharged thereafter (curve 3), the discharge voltage drops compared to the initial characteristic obtained when the battery was discharged adequately (curve 1). The discharge profile shows a two-stage change, and the battery does not regain its original capacity even when discharged to the original end voltage.
This phenomenon is known as "voltage depression." Sometimes the battery may appear to remember the low capacity at the time of a shallow discharge, so it is also called the "memory effect." This type of loss is accelerated as the temperature becomes higher.
The drop in capacity due to such a memory effect is different from substantial deterioration in that the original state can be restored by performing several complete discharge/recharge reconditioning cycles.
Therefore, as countermeasures against the memory effect, usage warnings to the effect that shallow charging and discharging should not be repeated are posted on products, and mechanisms are provided which occasionally perform deep discharges (to the completely discharged state) and return the battery to its initial state. However, each of these countermeasures is inconvenient for the user.
In addition, with the appearance of secondary batteries made of new materials, namely lithium ion batteries and lithium polymer batteries and the like, these batteries do not have the disadvantage of the memory effect seen in nickel-cadmium batteries and nickel-hydrogen batteries because they do not use the nickel anode said to be the main cause of the memory effect. However, these secondary batteries made of new materials do not have a long history of practical use, their safety is not adequately established, and additional circuitry is required for these countermeasures, and thus they have many disadvantageous aspects such as packaging aspects and cost aspects. In addition, there is an additional problem in that the cost of the cells themselves is higher than that of the conventional nickel-cadmium batteries and nickel-hydrogen batteries.